My code sample is:
private String _body;
private String _forward;
// setup message body
BodyPart messageBodyPart = new MimeBodyPart();
messageBodyPart.setContent(_body + "\n" + "--------------------------------" + "\n" + _forward , "text/html");
_multipart.addBodyPart(messageBodyPart);
When I send this message to my email address it shows:
_body = “Yeni Yazi”
and _forward = “deneme mail 2”

Since you’re sending email as HTML text you should use
<br>instead as that’s the HTML line break.